What Temperature Should Air Be Coming Out Of Vent When Cooling?

If you’ve always wondered how cold the air coming out of the mini split air conditioner should be, you’re not alone. It’s a question many homeowners ask, especially when it feels like the air isn’t cold enough.

And, it’s warranted too. Although it’s rarely a big issue for newer air conditioners, older units tend to produce not-so-cold air that may leave you wondering whether the unit is broken. Or worse, the appliance may be indeed compromised or broken.

A broken air conditioning unit can do much worse than leaving in the biting cold. For instance, you may need to replace the entire air conditioning unit. Or you may even experience water damage issues.

This guide answers the big question: What should the air temperature come out of the vent when cooling be?

We also explain how to measure the temperature of the supply air, the temperature difference, and determine whether your air conditioner is working correctly. Above all, we discuss several issues that may cause abnormal cooling and fix them.

What Should Be the Temperature of the Air Coming Out of the Vent when Cooling?

Generally, the temperature at the supply vent when cooling with an air conditioner should be 16°F to 22°F lower than the air temperature entering the air conditioning unit from your rooms. So, if the hot air returning to the air conditioning unit is 85°F, the air coming out of the supply vent should be 63°F to 69°F.

How Air Conditioners Work

Air conditioners lower indoor temperatures by gradually and continually removing heat from the air inside your home.

It’s a pretty straightforward process that begins when your thermostat requests cooling, i.e., when indoor temperatures rise above the thermostat setting, typically 78°F.

The air conditioning system will initiate a refrigeration cycle that moves cold refrigerant to the indoor unit. 

Simultaneously, the indoor air handler draws the hot indoor air into the unit. This hot air is forced over the cold refrigerant coils, allowing the refrigerant to absorb the most heat.

Then the now-cold air returns to the house, effectively lowering indoor temperatures. Meanwhile, the now-hot refrigerant flows back to the outdoor compressor, where it’s compressed to remove the heat.

The heat is disposed to the outdoor air while the cold-again refrigerant flows back to the indoor air handler to extract even more heat.

The process goes on and on until the thermostat setting is reached. Then it goes off until the thermostat requests cooling again.

Understanding Return vs. Supply Air

To understand temperature changes during the cooling process, you need to grasp two important terms – return air and supply air – and their difference.

What is Return Air?

The air “returning” to your air conditioner for “re-cooling” is known as return air. It returns to the air conditioner because it has lost its “conditioned” status and must be “re-conditioned” to maintain the desired indoor air conditioners.

The easiest way to memorize return air is to remember that cold air originates from the air conditioner. Thus, not-so-cold air must “return” to the air conditioner for re-cooling.   

What is Supply Air?

The air leaving your air conditioner and entering your home is known as “supply” air. It is known as supply air because the air conditioner’s main job is to provide “or supply” conditioned/cool air to the rooms.

So, an easy way to remember what “supply air” means is to recall again that the AC is designed to provide or “supply” cold air to your home.

Calculating “Delta T”

Now that you understand the concepts of supply and return air, you can determine the difference between the two to find out if your air conditioner is working accurately.

What is “Delta T?”

Delta T, or ΔT, stands for “change in” temperature. Delta is a Greek symbol that means “change in,” while the capital “T” stands for temperature.

The term means the difference between return air temperature and supply air temperature in air conditioning.

In other words, it stands for the change in temperature between the air entering the air conditioner from your home and that entering your home from the air conditioner.

How to Measure Temperature Differential (Delta T)

It’s very easy to measure ΔT. All you need is a temperature meter or dry bulb thermometer. First, measure the return temperature reading at the return air duct, about a foot from the duct, and note the figure.

Assign the value T1. Then, measure the supply temperature at the supply duct/air outlet, again about a foot from the outlet, and record the figure. We’ll assign this value T2. 

ΔT= T1 – T2. 

So, for instance, if T1 = 80°F and T2 = 65°F;

ΔT = 80°F – 65°F = 15°F. 

What’s the Ideal Delta T?

The ideal Delta T for an air conditioner is between 16°F and 22°F. However, for heat pumps, the ideal Delta T is high as 30°F.

When Delta T is Too High (More than 22°F)

A very high Delta T is often a sign of low indoor airflow. Low airflow causes difficulty moving hot air from the other end of the room to the AC intake vents for cooling. Alternatively, it could signal a clogged air filter or a damaged blower fan. Or worse, it could be a damaged compressor.

When Delta T is Too Low (Less than 16°F)

If the Delta T is too, it’s often an indication of low refrigerant. If the refrigerant levels drop, the air conditioner cannot remove heat at the expected rate because of diminished “carrying capacity.” Other reasons for low Delta T are outdoor unit fin deterioration and low outdoor airflow. Poor outdoor airflow can cause the outdoor air to reject heat from the air conditioner.

My Air Conditioner is Blowing Warm Air: Causes and Fixes

If your air conditioner is blowing warm, it means that the temperature difference (Delta T) is either too little or in the negatives, meaning the supply air is warmer than supply air. This may sound strange, but it’s possible. An air conditioner can blow warm for the following five main reasons;

Dirty Filter

Dirty filters blow hot indoor air from entering the air conditioner. This can usually result in very little cooling or even a warm air supply.

Solution: The only solution is to replace the dirty filter. More importantly, though, ensure regular maintenance as the manufacturer recommends.

Frozen/Dirty Evaporator Coils

A layer of frost or dust/dust over the evaporator coils reduces or completely removes the surface area for heat exchange, thus little cooling, usually resulting in the lukewarm supply of air.

Solution: Regular maintenance can help prevent both issues. Additionally, make sure to schedule professional maintenance at least once a year. If the problem is already ongoing, call an HVAC professional.

Incorrect Thermostat Setting

Have you confirmed that the thermostat is set to cool? This is important as you may unknowingly leave the AC on FAN mode. Or worse, you may have accidentally set your unit to HEAT if it’s a dual-purpose air conditioner and heat pump.

Solution: Make sure the air conditioner is set to COOL and the operational mode is AUTO. This way, the fan will only blow if the unit is cooling.

Electrical Power Challenges

Both the outdoor unit and indoor air handler must be powered and working correctly for the air conditioner to blow cool air. If the outdoor unit is disconnected while the indoor unit continues to run, you’ll get warm supply air.

Solution: Make sure indoor and outdoor units are both properly wired and that both are getting power. Otherwise, call your HVAC technician.

Refrigerant Leaks

Finally, you may also want to check for refrigerant leaks. Low or leaking refrigerant reduces the capacity of the air conditioner to extract heat from indoor air. Sometimes the problem can be so bad that the unit begins to blow lukewarm air.

Solution: It’s best to let a professional HVAC technician handle refrigerant issues as freon is a health risk. However, there’s no harm in keeping an eye out for signs of low refrigerant, including higher energy bills and hissing/bubbling noises.

AC Not Blowing Air At All! Causes and Fixes

If you’re not getting air completely, you may have an even bigger problem. First, make sure the air conditioner. If so, diagnose it for three possible issues;

Broken Fan Motor

The fan motor is responsible for drawing warm air into the air conditioner for cooling. Thus, you won’t get any air, even lukewarm air, if the fan motor is broken.

Fortunately, you can repair the motor. Alternatively, consider replacing it. Unfortunately, it costs $300 to $600 to replace the motor.

Blocked Filter, Registers, or Vents

The filter, registers, and vents (return or supply) can become blocked with solid particles, dust, and so forth.

When this happens, airflow through the air conditioner may be completely curtailed, meaning you may not get any air at the supply vents. Regular maintenance can fix these issues. However, don’t hesitate to contact an HVAC technician.

Blocked Condenser Coils

Finally, you may also experience intense icing that blocks the condenser coils. This will automatically also block air from flowing through to the other end. Remember that such intense icing is dangerous for you and the AC.

It can cause refrigerant line bursts that are a health hazard. At the same time, it can completely damage the air conditioner, necessitating a new unit altogether.

What Temperature Should Air from AC Be FAQs

Why is my Air Conditioner Blowing Warm Air?

If your air conditioner is blowing warm air, your refrigerant levels are likely low, a common sign of a refrigerant leak. Alternatively, you may have blocked air filters or frozen or dirty evaporator coils.

Why is my AC ON but not Cooling?

If your air conditioner is running but not cooling, you may have blocked or clogged condenser coils. Alternatively, the filter could be blocked/dirty, or you may have low refrigerant levels due to a refrigerant leak.

Why did my AC Suddenly Stop Working?

If your air conditioner suddenly stops working, it may be something as small as a tripped circuit breaker, blown fuse, or as bad as a damaged compressor. Check if it’s getting power and if so, call your HVAC technician.

How do I Reset my air Conditioning Unit?

You can reset your air conditioner in a few simple steps. Begin by powering the AC (flipping the circuit breaker), then locate the AC reset button, hold it down for 3-5 seconds, then release it. After that, restore power to the AC and turn it ON. That’s all.

What to Check if AC is not Working?

If your AC isn’t working, you can check a few things to troubleshoot the problem. Begin by checking if it’s getting power at the circuit board. Then make sure the thermostat is working and set correctly. From there, check the switches and overloads, including the condensate overflow switch. Finally, make sure the capacitor and contractor in the compressor are working. Beyond that, you should contact an HVAC technician.


So, there you have it. When cooling, the air coming out of your air conditioner should be 16°F to 22°F lower than the air entering the air conditioner. This difference is known as the Delta T (ΔT). A higher or lower Delta T value is often a sign of trouble.